THE  OVERVOLTAGE  OF  CHLORINE 

BY 

GRACE  GREENWOOD  SPENCER 


THESIS 

FOR  THE 

DEGREE  OF  BACHELOR  OF  SCIENCE 


CHEMICAL  ENGINEERING 


COLLEGE  OF  LIBERAL  ARTS  AND  SCIENCES 

UNIVERSITY  OF  ILLINOIS 


1922 


) 922 

-5p  3 


UNIVERSITY  OF  ILLINOIS 


Al§'Y._lAj 192.^  •_ 

THIS  IS  TO  CERTIFY  THAT  THE  THESIS  PREPARED  UNDER  MY  SUPERVISION  BY 

Grace  Greenwood  Spencer 

ENTITLED  j-Le  Cvervcl'tap  e 1 


IS  APPROVED  BY  ME  AS  FULFILLING  THIS  PART  OF  THE  REQUIREMENTS  FOR  THE 
DEGREE  OF  Bachelor  of  Science  in  Chemical  Engineering. 


ACTING  HEAD  OF  DEPARTMENT  OF  --CHEMISTRY 


500188 


Digitized  by  the  Internet  Archive 
in  2016 


https://archive.org/details/overvoltageofchlOOspen 


Acknowledgement 

The  completion  of  this  research 
ha3  been  made  possible  by  the 

unfailing  kindness  and  helpful 

suggestions  of  Doctor  J.  H.  Reedy, 

to  whom  the  writer  wishes  to 

egress  her  sincere  gratitude  and 

appreciation  for  hi3  assistance  in 

both  the  experimental  work  and  in 

the  writing  of  this  thesis. 


Table  of  Contant3. 

Pag 

I.  Introduction  1 

A.  Statement  of  the  Problem  1 

B.  Methods  of  Measurement  .1 

C.  Definition  of  Overvoltage  3 

D.  Review  of  the  Theories  of  Overvoltage  ...4 

E.  Methods  for  Determining  Overvoltage  .....5 

II.  Experimental  7 

A.  Apparatus  7 

B.  Procedure  8 

C.  Results  with 

1.  Smooth  Platinum . 9 

2.  Platinized  Platinum 9 

3.  Copper  . 10 

4 . Carbon  10 

5.  Gold  .11 

III.  Discussion  of  Results  12 

A.  Effect  of  Concentration  ..12 

B.  Effect  of  Surface  12 

C.  Effect  of  Time  12 

D.  Effect  of  Current  Density  ,...13 

E.  Mechanism  of  the  Reaction  14 

F.  Comparison  with  Newbery's  Results  16 

G.  Comparison  with  the  Overvoltage  of 

Hydrogen  and  Oxygen  17 

IV.  Summary  ...17 


A Study  of  the  Overvoltage 
of  Chlorine . 

I • Introduction. 

The  purpose  of  this  investigation  w as  to  determine  the 
factors  which  influence  the  overvoltage  of  chlorine  and  how  much 
each  factor  influences  this  overvoltage.  In  spite  of  the  fact 
that  thi3  phenomena  is  well  known  and  that  the  knowledge  that  the 
overvoltage  of  hydrogen  and  of  oxygen  have  many  useful 
applications,  no  exact  agreement  has  been  reached  as  to  the  best 
method  of  measuring  it. 

Two  radically  different  methods  have  been  used,  namely: 

1.  The  commutator  method,  in  which  the  current  is 
periodically  interrupted  by  means  of  a commutator  devise  while 
the  readings  are  being  made. 

2.  The  direct  methou,  in  which  the  exciting  current  remains 
flowing  while  the  potentiometer  readings  are  taken. 

These  two  methods  of  measurement  have  been  the  subject  of 
much  discussion,  especially,  between  Newbery  (l)  and  the 
American  chemists.  The  commutator  method  is  used  by  Newbery  almost 
exclusively,  while  the  direct  method  has  been  used  by  the 


(1).  J.  Chem.  Soc.,  165,  2420  (1314);  109,  1051  (1916); 
119,  477  (1921). 

J.  Am.  Chem.  Soc.,  42,  2007  (1920). 


2 

American  chemists.  Until  recently  no  attempt  has  been  made  to 
compare  these  two  methods.  Tartar  and  Keyes  (l)  recently  have 
published  an  article,  in  which  they  compared  the  results  obtained 
by  determining  overvoltage  by  the  two  methods.  As  a result  of  the 
discussion  of  these  methods,  there  has  been  some  controversy  over 
what  is  meant  by  the  term  overvoltage. 

According  to  Newbery  (2)  "the  potential  of  an  electrode  at 
which  molecular  hydrogen  is  being  formed  from  hydrogen  ions 
depends  not  only  upon  the  overvoltage  but  also  upon  the  applied 
electromotive  force,  the  resistance  of  the  electrode,  and 
especially  upon  the  transfer  resistance. 

"Overvoltage  is  of  necessity  an  active  voltage  or 
electromotive  force,  capable  of  doing  work  after  all  external 
sources  of  electromotive  force  have  been  removed.  Transfer 
resistance  is  a purely  passive  or  frictional  force  which  entirely 
(Ceases  when  the  external  electromotive  force  is  interrupted.  These 
forces  work  in  the  same  direction  during  the  passage  of  the 
current;  but  directly  this  current  is  removed,  the  overvoltage 
alone  exists  and  may  then  be  measured.  Although  overvoltage  and 
transfer  resistance  occur  together  in  this  way,  they  are  mutually 
independent  and  frequently  vary  in  opposite  directions." 

According  to  Mac  Innes,  on  the  other  hand,  "Newbery* s 
definition  of  the  term  overvoltage  should  be  reserve d for 
potentials  determined  by  a commutator  devise,  which  periodically 
opens  the  exciting  current  at  the  electrode  under  examination 

(1).  J.  Am.  Chem.  Soc.,  44,  557  (1922). 

(2^.  J.  Am.  Chem.  Soc.,  42,  2007  (1920). 


3 

and  closes  the  potentiometer  circuit  connecting  the  electrode  and 

a reference  electrode.  In  the  work  of  practically  every  other 
person  in  this  field,  the  exciting  current  remains  flowing  while 
the  potentiometer  measurements  are  made . " (l) 

The  chief  objection  to  the  commutator  method  is  that  the 
values  when  obtained  are  net  practical,  as  they  are  not  obtained 
under  conditions  which  will  exist  when  overvoltage  measurements 
are  of  value.  In  fact,  the  value  obtained  when  the  exciting 
current  is  flowing  is  undoubtedly  the  "working"  potential  and  the 
only  one  which  is  of  practical  value. 

In  spite  of  the  controversy  which  has  arisen  over  the 
method  of  measuring  overvoltage,  it  is  now  generally  agreed  that 
cyvervoltage  is  the  electromotive  force  which  acts  counter  to  the 
applied  electromot i ve.  force  during  electrolysis.  In  this 
investigation  the  overvoltage  of  chlorine  is  the  excess  of 
jsctential  that  exists  between  the  anode  at  which  molecules  of 
chlorine  gas  are  being  formed  from  the  solution  containing 

chlorine  ions  over  that  of  an  anode  whose  metal  surface  is 
saturated  with  chlorine  gas.  Or,  in  other  words,  chlorine 
overvoltage  is  the  difference  of  potential  between  a standard 
electrode  and  an  electrode  in  the  same  solution  at  which  molecules 
of  chlorine  gas  are  being  formed  from  a solution  of  hydrochloric 
acid.  This  may  be  better  illustrated  by  the  diagram  in  figure  1. 

"The  overvoltage,  therefore,  represents  the  excess  energy 
required  to  form  a substance  over  that  given  by  the  re-solution 


(1).  J.  Am.  Chem.  Soc.,  42,  2233  (1920). 


cpt  hod  £ 


anode 


u: 


<u 


COi 

' o m & / 

Bat 

ynatec/  uj///i  C//bs?^ 

4 


of  the  product  formed  to  the  original  state."  In  other  words,  "it 
is  the  amount  of  energy  by  which  one  measures  the  irreversibility 
of  the  process."  (l) 

There  are  several  theories  which  have  been  advanced  from  time 
to  time  to  explain  the  occurrence  of  overvoltage,  of  which  the 
following  are  the  most  important  and  widely  known: 

1.  That  the  electrode  adsorbs  a film  of  gas,  thus 
increasing  the  resistance,  a view  originally  suggested  by  Haber  (2) 
This  theory  has  been  discarded  since  it  was  learned  that  a 
resistance  at  the  electrode  would  lower  the  electromotive  force  of 
the  polarized  cell,,  and  also  that  metals  whlie  being  deposited  have 

definite  overvoltages  which  must  be  considered  aside  from  any 
gas  formation. 

2.  That  an  increased  solution  of  hydrogen  in  the  electrode 
is  caused  by  the  relative  slowness  with  which  the  electrode 
charged  with  gas  ean  get  into  equilibrium  with  the  atmosphere. 

This  view  was  advanced  by  Nernst  (3)  who  considered  that  before  the 
gas  can  be  liberated  the  ions  mu3t  be  driven  into  the  electrode. 
Metals  which  have  only  a slight  tendency  to  occlude  gases 
require  energy  to  force  the  gas  into  the  electrode,  and  thus 

give  high  overvoltages.  Tafel  (4)  takes  a similar  view  to  Nernst, 

in  that  he  considered  the  forcing  of  the  gas  into  the  electrode 
takes  time,  and  the  excess  gas  accumulated  generates  the  back 

(1) .  Trans,  Am.  Elect.  Chem.  Sec.,  29,  269  (1916). 

(2) .  Zeit.  Flek.  Chem.,  8,  539  (1902). 

(3) .  Ber. , 30,  1547  (1897 ). 

(4) .  zeit.  phys.  Chem.,  34,  300,  (1200). 


f ■ 1 


5 


electromotive  force.  He  also  considers  that  in  the  case  of 
hydrogen  an  intermediate  state  exists  between  the  dissolved  ion 
and  the  gaseous  hydrogen.  This  monatomic  hydrogen  forms  free 
hydrogen,  more  or  less  rapidly,  depending  on  the  catalytic  effect 
of  the  electrode.  When  the  reaction  is  slow  and  the  concentration 
of  the  intermediate  product  high,  the  overvoltage  is  high  and 
vice  versa.  The  theory  which  accounts  for  the  overvoltage  of 
hydrogen  as  being  due  to  monatomic  hydrogen  was  suggested  by 
Ostwald  (l)  and  Muller  (2),  later  accepted  by  Lewis  and 
Jackson  (3),  finally  by  Eancrcft  (4),  and  Rennet  and  Thompson  (5), 

3.  That  the  variation  in  the  surface  tension  of  large  and 
small  bubbles  of  gas  with  which  the  electrode  is  in  equilibrium 
causes  the  overvoltage  is  a theory  advanced  by  Helmholz  (6)  and 
Moller  (7)  and  recently  supported  by  Mac  Innes  and  Adler  (8). 

4.  That  the  formation  of  unstable  hydrides  cause  overvoltage 
is  a theory  advanced  by  Foerster  (9)  and  later  supported  by 
Newbery  (10).  Foerster  claims  that  at  a platinum  anode,  an  oxide 


(1) .  Zeit,  Elek.  Chem. , 6,  40  (1899), 

(2) ,  Zeit.  Anorgan.  Chem.,  26,  11  (1901 ). 

(3) .  Proc.  Arc.  Acad.,  41,  399  (1906). 

(4) .  J.  Phys.  Chem.,  20,  396  (1916). 

(5) ,  J.  Phys.  Chem.,  20,  296  (1916). 

(6) .  Thecrie  der  Warme  p.  309. 

(7) .  Zeit.  Phys.  Chem.,  65,  226  (1909). 

(6).  J,  Am.  Chem.  Soc.,41,  194  (1919).  (1909). 

(S).  Zeit., .Elek.  Chem. ,16,  353  (1910);  Zeit. Phys.  Chem., 69, 236 
(10).  J.  Chem.  Soc.,  105,  2420  (1914);  109,  1051  (1916), 


N-_. 


6 


probably  PtC>3  is  formed.  This  as  a solid  solution  wdiuld  generate 
a high  back  electromotive  force , and  therefore  give  a high 
oxygen  overvoltage  at  a platinum  anode.  In  the  case  of  other 
metals  -where  the  higher  oxide  is  unstable,  it  would  not 
accumulate  in  appreciable  quantities  and  hence  the  overvoltage 
wouio  be  low.  Hydrogen  overvoltage  would  be  explained  by  the 
assumption  of  the  formation  of  a solid  solution  of  the  hydride. 
This  theory  is  probably  true  for  a large  number  of  cases  but  the 
formation  of  these  hydrides  is  probably  a consequence  cd  and 
net  an  explanation  of  overvoltage.  Also  this  theory  does  not 
explain  the  cvervoitageof  the  metals. 

Aside  from  the  two  distinct  methods  of  measurement  of  the 
overvoltage,  there  ere  two  generally  accepted  methods  for 
determining  this  overvoltage.  Either  method  of  measurement  may  be 
employed  depending  upon  what  the  experimenter  understands  by  the 
term  overvoltage.  These  two  generally  accepted  methods  are: 

1.  The  bubble  formation  method,  that  is  , the  potential  as 
opposed  to  the  rate  of  formation  of  the  bubbles  if  gas. 

2.  The  current  density  method,  that  is,  the  potent!?.  1 as 
opposed  to  the  density  of  the  exciting  current. 

The  current  density  method  is  the  one  used  in  this 
particular  investigation.  It  was  assumed  in  this  method  of 
measurement  that 

1.  Neither  the  size  or  the  shape  of  the  containing  vessel 
nor  the  distance  between  the  ancae  and  the  cathode  should  exert 
any  influence. 

2.  The  value  found  must  represent  an  electromotive  force 
and  not  a resistance;  this  involves  the  elimination  of 


c 


. 

* 


7 

appreciable  potential  drop  due  to  the  resistance  of  an  electrode 
or  to  any  resistance  of  whatever  nature  at  the  surface  of  the 
electrode  (l). 

I I . Experimental . 

The  purpose  of  this  investigation  was  to  determine  the 
effects  of  the  anodes  of  copper,  of  gold,  of  smooth  platinum, of 
carbon,  and  of  platinized  platinum  on  the  overvoltage  of  chlorine 
from  solutions  of  normal,  tenth  normal,  hundredth  normal,  and  in 
some  cases  thousandth  normal  hydrochloric  acid  ; and  to  compare 
these  results  as  far  as  possible  with  those  obtained  by  Newbery  in 
which  he  used  the  commutator  method  (2).  Current  densities  varying 
from  zero  to  ten  milliamperes  were  used  in  nearly  every  case. 
Nothing  higher  than  ten  milliamperes  is  recorded  in  the  tables 
because  the  bend  in  the  curves  was  in  each  case  found  to  occur  at 
very  low  current  densities,  nothing  higher  than  four  milliamperes. 

The  apparatus  consisted  of  a Leeds  and  Ncrthrup 
potentiometer,  a D'arsonval  galvanometer  of  the  Leeds  and 
Northrup  type,  a lead  storage  cell,  an  Edison  cell,  a resistance 
bcx,  a slide  wire  resistance,  a small  galvanometer  standardized 
to  measure  fractions  of  a rcill.iarope.re , a mill iarome ter,  a 
standard  calomel  electrode,  and  an  electrolytic  cell.  The 
electrolytic  ceil  consisted  of  a stationary  bright  platinum 

cathode  and  a stationary  anode.  The  acid  contained  in  the  cell 

(1) .  J.  Am.  Cherc.  Soc.,  44,  557  (1922), 

(2) .  J.  Chem.  Soc.,  113,  477  (1921). 


8 


/\  mechanically  stirred  by  means  of  a winged  glass  stirrer. 

The  arrangement  of  the  apparatus  consisted  of  an  electrolytic 

and 

/potentiometer  circuit.  In  the  electrolytic  circuit  an  Edison  cell 

was  used  to  generate  the  exciting  current,  the  density  of  which 

was  varied  by  means  of  the  slide  wire  resistance.  The  small 

galvanometer  was  used  fdr  measuring  the  value  of  thfe  exciting 

was 

current  until  one  half  of  a milliampere  reached,  all  higher 
values  are  measured  on  the  milliamme ter . An  intermediate  vessel  of 
saturated  ammonium  nitrate  was  used  to  eliminate  as  far  a3 
possible  the  liquid-liquid  potential.  In  the  potentiometer 
circuit  a standard  cadmium  cell  was  used  in  order  to  regulate  the 
exciting  current  generated  by  the  lead  storage  cell.  The 
arrangement  of  the  apparatus  i3  shown  in  figure  2. 

By  this  arrangement  the  change  in  the  overvoltage  as  the 
density  of  the  exciting  current  was  increased  can  be  measured 
directly.  Care  was  exercised  so  that  the  readings  in  each  case 
were  taken  as  rapidly  and  a3  uniform  as  possible  in  order  to  have 
a uniform  basis  for  comparison  of  the  results.  The  change  in 
current  density  was  a gradual  advance  and  in  no  case  was  it 
changed  from  a higher  to  a lower  value  to  take  any  reading.  The 
results  obtained  are  listed  in  the  following  tables. 

Table  1 shows  the  change  of  the  overvoltage  of  chlorine 
using  a smooth  platinum  anode  from  concentrations  of  hydrochloric 
acid  equal  to  0.900  normal,  0.096  normal,  and  0.014  normal  acid 
with  a change  in  current  density  from  zero  to  ten  milliamperes . 

In  a similar  manner  table  2 shows  the  change  on  platinized 
platinum,  table  3 on  copper,  table  4 on  carbon,  and  table  5 on 
gold. 


. 

• 

CM 

I 

0? 


9 


Table  1 Smooth  Platinum. 


0.900  N Hcl 


current  overvoltage 
m.a.  volte. 


0.1 

0.8278 

0.2 

0.8385 

0.3 

0. 8453 

0.4 

0.851? 

0.5 

0.8556 

0.6 

0.8606 

1.0 

0.8723 

2.0 

0.8850 

3.0 

0.8939 

4.0 

0.9000 

5.0 

0.9064 

6.0 

0.9123 

7.0 

0.9156 

8.0 

0.9190 

9.0 

J . 9 <329 

10.0 

0.9258 

0.900  N HC1 

current 

overvoltage 

m.a. 

volta . 

0.1 

0.9250 

0.2 

0.9386 

0.3 

0.9449 

0.4 

0.9498 

0.5 

0.9546 

0.6 

0.9569 

1.0 

0.9665 

2.0 

0.9752 

3.0 

0.9822 

4.0 

0.9872 

5.0 

0.9912 

6.0 

0.994? 

7.0 

0.9969 

8.0 

1.0000 

9.0 

1.0030 

10.0 

1.0055 

0.096  N HC1 


current 

overvoltage 

m.a. 

volta . 

0.1 

0.9517 

0,2 

0.9624 

0.3 

0.9738 

0.4 

0.9803 

0.5 

0..  9865 

0.6 

0.-9942 

1.0 

1.0098 

2.0 

1.0318 

3.0 

1.0452 

4.0 

1.0604 

5.0 

1.0740 

6.0 

1.0850 

7.0 

1.0915 

8.0 

1 . 0945 

9.0 

1.1036 

10.0 

1.1105 

Table  2 

Platinized 

0. 

096  N Hrjl 

current 

overvoltage 

m.a. 

vclta. 

0,1 

0.9919 

0.2 

1.0036 

0.3 

1.0117 

0.4 

1.0134 

0.5 

1.0168 

co 

o 

1,0203 

1.0 

1.0336 

2.0 

1.047? 

3.0 

1.0580 

4.0 

1.0640 

5.0 

1.0703 

6,0 

1.0765 

7.0 

1.  :847 

8.0 

1.0896 

9.0 

1.0944 

10.0 

1.1020 

0.014  N HC1 


current 

overvoltage 

m.a. 

vclta . 

0,1 

1.1346 

0.2 

1.2085 

0.3 

1 . 2553 

0.4 

1.2793 

0.5 

1.2963 

0.6 

1.3094 

1.0 

1.3518 

2.0 

1.3S32 

3.0 

1.4048 

4.0 

1.4185 

5.0 

1.4298 

6.0 

1.4422 

7.0 

1.4513 

8.0 

1.4590 

9.0 

1.4693 

10.0 

1.4760 

tinum. 

0.014 

N HOI 

current 

overvoltage 

m.a. 

volta . 

0.1 

1,0335 

0.2 

1,0558 

0.3 

1 . 0670 

0,4 

1.0708 

0.5 

1,0770 

0.6 

1.0822 

1.0 

1.0946 

2,0 

1.1002 

3.0 

1.1094 

4.0 

1.1150 

5.0 

1.1195 

6.0 

1.1287 

7.0 

1.1341 

8.0 

1.1480 

9.0 

1.1504 

10.0 

1.1556 

Volfs 


/ 


MiUiamperes 


10 


Table  3 Copper 


0.935 

N HC1 

0.107 

N HC1 

0.013 

N HC1 

current 

overvoltage 

current 

overvoltage 

current 

overvoltage 

m.a. 

volt  9 . 

m.a . 

volts . ~ 

rn.  a. 

volts'^ 

0.11 

-0.4146 

0.11 

-0.2749 

0.10 

-0.1154 

0.18 

0.4056 

0.18 

0.2607 

0,20 

0. 1094 

0.28 

0.3932 

0.30 

0.2459 

0.30 

0.1016 

0.46 

0.3810 

0.39 

0.2400 

0.44 

0.0937 

1.00 

0.3521 

1.00 

0.3284 

0.60 

0.0833 

2.00 

0.3336 

2.00 

0.3100 

1.00 

0,0459 

3.30 

0.3207 

3.00 

0.1960 

2.00 

0.0071 

4.00 

0.3149 

4.00 

0.1806 

3.00 

+ 0 . 0523 

5.00 

0.3106 

5.00 

0.1707 

4.00 

0.0879 

6.00 

0.3036 

6.00 

0.1612 

5 . 00 

0.1233 

7.00 

0.3000 

7 . 00 

0.1517 

6.00 

0.1641 

8.50 

0.2S37 

8.00 

0.0775 

7.00 

0.2004 

10.00 

0.2799 

9.00 

10.00 

0. 0667 
0.0526 

8.00 

9.00 

10.00 

0.2405 

0.2820 

0.3239 

Table  4 Ga 

rbon 

0.914 

N HC1 

0.102  N 

HC1 

0.010 

N H01 

current 

overvoltage 

current 

overvoltage 

current 

overvoltage 

m.a. 

volts . 

m.  a. 

volts . 

m.a. 

volts . 

0.05 

0.3452 

0.10 

0.6826 

0.08 

0.7605 

0.18 

0.3642 

0.18 

0.6929 

0.20 

0.7660 

0 . 26 

0.3817 

0.28 

0.6953 

0.30 

0.7776 

0.40 

0.4569 

0.43 

0.7072 

0.  40 

0.7856 

0.48 

0. 4832 

1.00 

0.7703 

0.56 

0.8242 

1.00 

0.5332 

1.50 

0.8473 

1.00 

0.9306 

2.00 

0.6615 

2.40 

0.9126 

2.00 

1.0334 

3.00 

0.7624 

3.50 

1.0091 

3.00 

1.1308 

4.00 

0.8223 

5.00 

1.0625 

4,00 

1.1846 

5.00 

0. 8690 

6.00 

1 . 0903 

5. 00 

1.2372 

6 . 00 

0.8986 

7.00 

1.1254 

6.  00 

1.2841 

7.00 

0.9267 

8.00 

1.1452 

7.00 

1.3307 

8.00 

0.9428 

9.00 

1.1745 

8.00 

1.3704 

9.00 

0.9724 

10.00 

1.1975 

9.00 

1.4049 

10.00 

0,9942 

10.00 

1.4535 

11 


Table 

5 Gold 

0.935 

N HG1 

0.107 

IT  HC1 

0. 

013  N HC1 

current 

ovsrvol tage 

current 

overvoltage 

current 

overvoltag 

m . a . 

volts . 

ra.  a. 

volts. 

m.  a. 

vcltsT 

0.00 

0,4377 

0.12 

0.6845 

0.03 

0.8331 

0 . 03 

0.4554 

0.32 

0.7253 

0,09 

0.8549 

0.06 

0.4745 

0,42 

0.7377 

0,23 

0.8748 

0,09 

0.4835 

0.52 

0.7493 

0.54 

0.8956 

0.10 

0,5228 

1.00 

0.7810 

1.00 

0.9223 

0.26 

0.5588 

2.00 

0.8052 

2.00 

0.9421 

0.38 

0.5880 

3.00 

0.8177 

3.00 

0.9530 

0.60 

0.6096 

4.00 

0.82.57 

4.00 

0.9605 

1.00 

0.6348 

5.00 

0. 8353 

5.00 

0.9698 

2 . 00 

0.6842 

6. 00 

0.8405 

6.00 

0,9601 

3.00 

0.7131 

7.00 

C . 8456 

7.00 

0,9899 

4.00 

0.7240 

8,00 

0.8500 

8.00 

0.9956 

5,00 

0.7324 

9.00 

0.8549 

9.00 

1.0036 

6.00 

0.7381 

10,00 

0 . 8605 

1 0,00 

1.0112 

7.00 

0,7421 

8,00 

0.7493 

9.00 

'0. 7525 

10.00 

0.7567 

0.0012 

N HOI 

current  overvoltage 

re.  a. 

volts . 

0.02 

0.8484 

0.03 

0.9156 

0.04 

0183 

0.06 

1,2265 

0*07 

1.4093 

0.14 

1,4650 

0.24 

1.4892 

0.34 

1.5081 

0.44 

1,5223 

0.60 

1,5396 

1,00 

1.5781 

2.00 

1,6238 

3.00 

1,6625 

4.00 

1.6886 

5.00 

1.7073 

miiamperes 


12 


III.  Discussion  of_  Results . 

The  results  of  this  experiment  are  similar  to  those 
commonly  known  for  hydrogen  and  oxygen,  namely,  that  the 
overvoltage  varies  with  the  concentration  of  the  acid  used  and 
with  the  electrode  surface.  A decrease  in  con  centration  of  the 
anion  or  an  increase  in  concentration  of  the  cation  of  the  anode 
metal  increases  .the  overvoltage .. This  was  found  to  be  true  for 
every  electrons  surface  used,  but  in  some  cases  the  variation 
was  greater  than  in  others.  In  regard  to  the  electrode  surface 

the  values  varied  from  cooper  with  a negative  value  of  about  0.42 
volts  in  normal  HC1  and  with  a current  density  of  0.1  m.a.  per 

square  decimeter,  through  carbon  with  a positive  value  of  0.35 
volts,  gold  with  a positive  value  cf  0.53  volts,  smooth  platinum 
with  a positive  value  cf  0.83  volts,  to  platinized  platinum  with 
a positive  value  of  0.S3  volts. 

In  spite  of  the  fact  that  both  Newbery  (l)  and  Tartar  and 
Keyes  (2)  consider  that  the  time  element  is  a very  important  item 
in  the  determination  of  the  overvoltage,  for  values  as  low  as 

those  used  in  this  experiment  and  where  the  time  involved  in  taking 
the  readings  is  as  short  as  it  was  in  these  cases  it  is  not 
necessary  to  consider  it.  Several  control  tests  were  run  in 
order  to  determine  the  effect  of  time  6n  the  overvoltage  under 
the  conditions  involved.  Although  the  time  effect  is  great  enough 
to  prevent  complete  agreement  in  the  values,  yet  the  variation 
was  not  great  enough  to  appreciably  effect  the  results.  Since  the 
effect  of  time  on  the  overvoltage  is  the  same  as  the  effect  of  a 

(1) .  J.  Chem.  Soc.,  119,  (1921). 

(2) .  J,  Am.  Chem.  Soc.,  44,  557  (1922). 


13 

decrease  in  concentration  of  the  aniens  or  an  increase  in 
concentration  of  the  cations  of  the  anode  metal,  it  seems  highly 
probable  that  this  increase  in  potential  which  has  been  attributed 
to  the  time  involved  is  very  likely  due  to  the  fact  that  chlorine 
is  being  given  off  from  the  solution  and  hence  lowering  the 
concentration  of  the  aniens  particularly  around  the  anode.  Several 
facts  point  to  this  conclusion,  first,  that  the  potential  increases 
in  every  case  with  a decrease  iij  concentration  of  the  anion  or 
with  an  increase  in  concentration  of  the  cation  of  the  anode  metal, 
second,  that  the  greater  the  current  density  the  more  rapidly  is 
the  potential  increased  in  a given  time0 

With  the  exception  of  the  results  obtained  with  the  carbon 
anode,  the  bending  point  in  the  curve  or,  in  other  words,  the 
point  of  maximum  overvoltage  occurs  before  a current  density  of  one 
riilliampere  per  square  decimeter  is  reached.  In  the  case  of  the 
smooth  platinum,  the  bend  occurs  at  about  four  tenths  of  a 
milliampere  when  normal  hydrochloric  acid  is  used,  and  at  eight 
tenths  when  hundredth  normal  acid  is  used.  With  the  platinised 
platinum  the  bend  occurs  at  about  the  seme  current  density  in  each 
of  the  varies  concentrations  used,  this  value  in  each  case  was 
between  two  and  three  tenths  of  a milliampere.  With  the  gold  the 
point  of  maximum  overvoltage  occurs  at  about  four  tenths  of  a 
milliampere  for  the  normal  acid,  at  about  five  tenths  with  the 
tenth  normal,  at  about  seven  tenths  with  the  hundredth  and  with 
the  thousandth  normal.  When  the  copper  anode  is  used  the  bend 
comes  at  four  tenths  of  a milliampere  in  each  case.  Due  to  the  fact 
that  copper  is  very  soluble  in  hydrochloric  acid  when  an  electric 
current  is  being  passed  through  the  ceil,  it  was  very  difficult  to 


14 


secure  checks  on  the  results  obtained.  Those  which  are  tabulated 
although  they  are  the  best  that  could  be  obtained  at  the  time  are 
not  very  reliable  as  very  many  different  kinds  of  results  were 
obtained  during  the  experimentation.  The  ones  given  were  selected 
because  they  seemed  to  be  the  nearest  to  the  average  and  because 
the  curves  which  represent  them  were  most  nearly  in  agreement 
Wj^th  the  results  obtained  with  the  other  anodes.  The  copper  anode 

was  a copper  wire  sealed  into  a piece  of  glass  tubing  with 
sealing  wax.  In  the  case  of  the  carbon  anode  used,  there  seemed 
to  be  quite  a divergency  in  the  results  obtained.  This  variation 
in  the  values,  unlike  that  of  copper,  tended  to  cause  an  irregular 
curve  rather  than  causing  several  different  kinds  of  curves  as 
wasffound  in  the  case  of  the  copper.  But  by  plotting  the  best 
results  obtained  it  was  found  that  the  bend  in  the  curve  occurred 
at  about  two  and  five  tenths  railliamperes  for  the  normal  acid,  and 

at  about  three  miliiamperes  for  the  tenth  and  the  hundredth 
normal  acids. 

Another  point  which  was  particularly  noticeable  was  that 
after  the  point  of  flexure  was  reached  the  more  rapidly  that  the 
readings  were  taken  the  more  nearly  the  curve  approached  the 
perpendicular,  indicating  that  if  it  were  possible  to  take  ail  the 
readings  under  absolutely  the  same  conditions  of  concentration 
that  after  the  flexure  point  was  reache'd  the  variation  of  the 
overvoltage  with  the  current  density  is  equal  to  zero. 

The  point  of  flexure  or  the  point  of  maximum  overvoltage 
seems  to  be  due  to  the  fact  that  at  that  point  chlorine  gas  is 
given  off,  probably  according  to  the  equation 

2 Cl'  + 2©  = Clg 


15 


In  the  case  of  the  copper  anode  which  in  some  cases  showed  a 
double  flexure,  the  second  point  of  inflection  is  probably  due  to 
the  going  into  solution  of  the  copper  ions.  For  example,  in  the 
case  of  the  copper  anode  in  the  tenth  normal  acid  solution  in 
the  interval  between  7.8  milliamperes  and  8,0  milliarcperes  the 
copper  plated  very  rapidly  upon  the  cathode.  This  showed  that  the 
copper  dissolved  from  the  anode  according  to  the  equation 

Cu  4 2 © - Cu"  (2) 

and  then  after  forming  the  copper  ions  in  solution  these  ions 
again  give  up  the  positive  charges  and  plate  onto  the  cathode 
according  to  the  equation 

Cu*  4 2©  s Cu  (3) 

In  every  case  where  this  plating  occired  it  did  not  start  until  a 
given  current  density  was  reached  and  then  it  plated  on  very  rapidly 
The  point  of  plating  was  also  the  point  of  inflection  in  the  curve. 
This  would  indicate  that  a certain  amount  of  energy  had  to  be 

stored  up  before  the  reaction  (3)  could  start,  but  after  it  had 

once  started  it  would  progress  very  rapidly.  This  explains  the 
occurrence  of  the  point  of  flexure  in  the  curve,  but  the  best 
explanations  of  the  actual  occurrence  of  overvoltage  are  probably 

those  given  by  Bennet  and  Thompson  (l)  and  Bancroft  (2))and 
Rideal  (3). 

Bennet  and  Thompson  conclude  that  any  chemical  reaction, 
consisting  of  more  than  one  step,  in  generating  electricity  can 
not  be  strictly  reversible,  but  requires  more  electrical  eitergy  to 
reform  the  substance  than  is  given  by  the  reverse  reaction.  This 

(1) ,  J.  Phya.  Chem. , 20,  236  (1316). 

(2) .  J.  Phys.  Chem.,  20,  396  (1916). 

(3) .  J.  Am.  Chem.  Soc.,  42,  94  (1920). 


16 

irreversibility  gives  rise  to  overvoltage,  since  the  quantity 
factor  is  constant.  The  theory  is  that  the  excess  of  back 
electromotive  force  of  the  system  during  electrolysis  over  the 

reversible  electromotive  force  of  the  system  consisting  of  the 
final  product,  is  due  to  the  accumulation  during  such  electrolysis 
of  unstable  intermediate  products  above  the  equilibrium 
concentration.  These  products  are  unquestionably  active  hydrogen, 

K^*  active  oxygen,  Op,  etc.  These  products  have  bean  shown  to  be 
more  reactive  than  the  final  products,  and  are  sufficently  active 
to  explain  the  overvoltages  found  experimentally, 

Rideal  explains  overvoltage  on  the  ground  of  the  adsorption 
theory,  namely,  that  it  is  the  measure  of  the  energy  required  for 
the  desorption  of  the  hydrogen  from  the  metallic  surface.  He  also 
considers  that  a monatomic  gas  is  formed,  which  influences  the 
overvoltage. 

In  comparison  with  the  results  obtained  by  Newbery  (l)  with 
the  commutator  method,  it  may  be  said  that  the  results  obtained  by  t 
the  direct  method  are  in  every  case  higher  in  value  than  the  ones 
obtained  by  the  commutator  method.  The  curves  are  not  very  much 
alike  either  as  to  general  shape  or  as  to  the  point  of  flexure.  In 
ties  first  place  the  point  of  flexure  obtained  by  Newbery  was  at  a 
much  wider  range  of  current  densities  than  those  used  in  this 
experimentation,  also  he  aid  not  tabulate  his  results  for  readings 
taken  as  frequently  as  those  obtained  in  this  work.  In  almost 
every  case  the  point  of  flexure  found  in  this  investigation 
could  not  be  found  from  the  readings  obtained  by  Newbery  as  his 
first  three  readings  were  taken  at  two,  four,  and  six  milliamperea . 

fl).  J.  Chem.  Soc.,  119,  477(1921). 


. 


. 

. 

. 

' 


. 

, 


. 


. . 


' 


, 


. 


17 


Another  fact  which  was  pointed  out  by  Tartar  and  Keyes  in  their 
work  in  comparing  the  results  obtained  by  the  direct  and  by  the 
commut&tbr  methods  of  measurement  of  overvoltage,  is  the  fact  that 
the  point  of  inflection  in  the  curve  is  approached  gradually  when 

measurements  are  made  by  the  direct  method  and  that  it  occurs 
abruptly  when  the  measurements  are  made  by  the  commutator  method. 

This  abrupt  change  would  indicate  that  the  potential  was  gradually 
increased  until  a critical  point  was  reached  where  the  chlcrina? 
wae  suddenly  evolved,  which  is  not  the  generally  accepted  opinion. 

One  radical  difference  exists  between  the  overvoltage  of 
chlorine  and  the  overvoltage  of  hydrogen  and  oxygen.  This  difference 
is  that  in  the  case  of  both  oxygen  and  hydrogen  the  overvoltage 
increases  with  the  polish  of  the  electrode  surface  used,  for 
example,  the  overvoltage  of  both  hydrogen  and  oxygen  on  smooth 
platinum  is  much  higher  than  that  on  platinized  platinum.  On  the 
other  hand,  the  overvoltage  of  chlorine  is  higher  on  platinized 
platinum  than  on  the  smooth  platinum.  No  satisfactory  explanation 
for  this  divergency  can  be  given  at  this  time. 

IV . Summary . 

1.  In  every  case,  a decrease  in  concentration  of  the  anion  or 
an  increase  in  concentrat ion  of  the  cation  of  the  anode  metal 
increases  the  overvoltage. 

2.  The  overvoltage  varies  with  the  electrode  surface  used, 
but  unlike  the  overvoltage  of  hydrogen  and  oxygen  it  doe3  not 
always  increase  with  the  polish. 

3„  The  effect  which  has  been  commonly  attributed  to  the  time 


. 


. 


. 


. 


. 


18 

is  here  regarded  to  be  due  to  the  change  in  concentration  close  to 
the  anode. 

4.  The  overvoltage  increases  with  an  increase  in  current  deneit 
density  very  rapidly  until  the  point  of  flexure  is  reached,  but 
after  that  only  very  slightly  if  at  all,  a limiting  value  is 
therefore  indicated. 

5.  The  overvoltage  is  due  to  the  formation  cf  the  molecules 
of  gas  from  the  ions  in  the  solution.  This  formation  is  not 
accomplished  in  one  step  but  involves  several  electrochemical 
reactions  and  is  therefore  an  irreversible  process. 

6.  The  results  obtained  did  not  agree  with  those  obtained  by 
Ne  vbery  either  in  the  absolute  value  or  in  the  general  shape  of  the 
curves  obtained. 


. - 


19 


V.  Bibliography. 

1.  Bennet  & Thompson:  Trans.  Far,  Soc.,  29,  269  (1916). 

2.  Richards:  Trans.  Far.  Soc.,  29,  140  (1916). 

3.  Hambueohen:  Trans.  Am.  Elect.  Chem.  Soc.,  18,  91  (1905). 

4.  Qrabtree:  J.  Soc.  Chem.  Ind. , 32,  521  (1913). 

5.  Newbery:  J.  Chem.  Soc.,  105,  2420  (1914);  109,  1051 ; (1916). 
119,47?  (1921);  J.  Am.  Chem.  Soc,,  42,  2007  (1920). 

6.  Tartar  & Keyes:  J.  Am.  Che^.  Soc.,  44,  557  (1922). 

7.  Le  Blanc:  Trans.  Far.  Soc.,  9,  218  (1896). 

8.  Rideal:  J.  Am.  Chem.  Soc., 42,  94  (1920). 

9.  Mac  Innes  & Adler:  J.  Am.  Chem.  Soc.,  41,  194  (1919); 

41,  2013  (1919). 

10.  Lewis:  J.  Chem.  Soc.,  105,  2331  (1914);  107,  233  (1915); 

109,  55  (1916);  109,  6?  (1916);  109,  796  (1916); 

111,  389  (1917);  111,  457  (1917);  111,  1086  (1917); 

113,  471  (1918);  115  , 182  (1919). 

11.  Lewis  & Jackson:  Proc.  Am.  Acad.,  41,  399  (1906). 

12.  Bancroft:  J.  Phys.  c^e^.»20,  396  (1916). 

13.  Bennet  & Thompson:  J.  Phys.  Chem.,  20,  296  (1916). 

14.  Foerater:  Zeit.  Electrochern.  , 16,  353  (1910), 

15.  Muller:  Zeit,  Anargan.  Chem.,  26,  11  (1901). 

16.  Foerstsr:  Zeit.  Phys,  Chem.,  69,  236  (1909). 

17.  Ostwald:  Zeit.  Electrochern.,  6,  40  (1899). 

18.  Mac  Innes:  J.  Am,  Chem.  Soc.,  42,  2233  (1920). 

19.  Haber:  Zeit.  Elek.  Chem.,  8,  539  (1902). 

20.  Nernst:  Ber.,  30,  154?  (1897). 

21.  Tafel:  Zeit.  phys.  Chem.,  34,200  (1900). 


